moon counts


Eric Johnson <e.johnson31@...>
 

All the trigonometry aside, it seems to me that the height of the birds will affect your count. Low birds passing
very close to you may be missed, high ones less often. After all, you have a cone of observation starting at your
scope and widening out at the moon. Maybe it's not enough to matter? If it does, then early evening counts
should be lower than later, since we know migrants tend to climb through the night (radar observations). And
does the angle at which you are observing affect your count? Seems to me you'd see more at low angles (moon
near the horizon) than directly overhead (or as direct as the moon can be). Any revelations from the geometers?

Eric


Brad Schram
 

Hi Eric:

Thanks for your thoughts. I'm happy to see the various responses because I
wonder about these things too--and I hope some others will give it a try.
It takes patience (and a willingness to have uncomfortable eyes for as long
as a few hours), but it's a marvelous thing to see.


All the trigonometry aside, it seems to me that the height of the birds
will affect your count. Low birds passing
very close to you may be missed, high ones less often.
Yes, this is true. I think the math (WARNING: any comments I make about
math are equivalent to a Chimpanzee commenting on French cuisine) takes this
into account, however, since it is based on the size of the moon and its
proportion to the entire observable sky from a given point.

The few birds that I've heard call in flight overhead while moon-watching
have all been invisible to me. So, yes, I'm missing thousands of birds
going by unseen at various levels--but that's why the math regarding a
sampling crossing the disk is intriguing.

Practically speaking, this is very obvious as you watch the moon. Some
birds, their silhouettes larger, flash by so quickly that their passing must
be measured in hundredths of a second. These, in my limited experience, are
in the minority however. Most are mid-range (whatever that means) to high
altitude (ditto) relatively speaking. There is usually time to gain a
decent impression of their flight characteristics and shape, thus my
comments regarding specific groupings into warblers, thrushes and
flycatchers etc.

I would be very interested in a mathematical rule of thumb which could tell
me, say: given the known dimensions of a warbler, how could its height be
APPROXIMATED by comparing it to the moon's dimensions? I know that the
moon's height, as you point out, changes the proportions and I have no way
to gauge it with precision. But we're talking art, not science, on this
point and it would simply be interesting to be able to guess an approximate
altitude for birds crossing the moon's diameter. I'm fully aware of the
imprecision resulting here, as I said--it's an art, not a science on this
point without sophisticated instruments--but it would be satisfying to have
some way to guesstimate an answer.

After all, you have a cone of observation starting at your
scope and widening out at the moon. Maybe it's not enough to matter? If it
does, then early evening counts
should be lower than later, since we know migrants tend to climb through
the night (radar observations). And
does the angle at which you are observing affect your count? Seems to me
you'd see more at low angles (moon
near the horizon) than directly overhead (or as direct as the moon can
be). Any revelations from the geometers?

Good thought on the ascending through time issue (and it would logically
follow that one would see more when the moon is close to the horizon since
one is looking through "more air"--unless a bird's invisibility due to size
and distance compensates for this). I've only conducted 3 moon watches,
each of different duration, each starting about 9:15. The only one
continuing to midnight showed a slight uptick in numbers later in the count.
I will have to watch for a "receding" effect, that is--higher birds as time
goes on.

I will also be interested to see if the number drops off after 2 AM, since I
read that passerines tend to land about that time. They (or a component of
them) are said to lift off again before dawn and continue for a few hours
(Frank Gill, "Ornithology, 2nd ed." p. 305ff). The phenomenon I see here,
night-time migrants against the moon and morning migrants flying by when
there is an east or northeast wind (rarely in a north wind) seems to fit
this pattern. I do need to do a watch from midnight until 4AM to test
this--on a night when I've already confirmed a decent flight between 9 and
10 PM. I'm not enthusiastic about the personal pain while doing this, but I
should do it simply to satisfy my curiosity.

All thoughtful comments welcome.

Brad